1. Field of the Invention
The present invention relates to a photomask having a desired flatness, a method of manufacturing such a photomask, and a method of manufacturing an electronic product.
2. Description of the Related Art
In recent years, various problems are becoming apparent in lithographic steps used in semiconductor manufacturing processes. As the dimensions of semiconductor devices have been scaled down, the demand has increased for finer pattern lithography. The device design rules have already scaled down to 0.13 μm. As a result of this, the pattern dimensions must be controlled with a very strict accuracy of the order of 10 nm.
Under such a circumstance, one factor preventing higher accuracy in pattern formation steps is the degrees of flatness of photomasks used in lithographic steps. With further shrinking of dimensions, the margin for the depth of focus in lithographic steps decreases. Under this circumstance, the degrees of flatness of photomasks has become non-negligible.
In general, the photomask is formed by depositing a translucent film and a shading film on a substrate of quartz. The translucent film has a large internal stress in excess of 1 GPa. This internal stress deforms the quartz substrate, deteriorating the flatness of the photomask. The deterioration of the flatness has constituted the prime cause of poor yield.
The translucent film and the shading film deposited on the substrate of quartz are etched into a desired mask pattern, whereby the photomask manufacturing process is completed. It is difficult to predict the deformation of the substrate after mask pattern formation from the internal stresses that the translucent and shading films have at the time of deposition; therefore, the flatness of the photomask cannot be controlled.